1. Field of the Invention
The present invention relates to transition metal boride coatings having excellent wear and corrosion resistance and to a process for preparing such coatings. More particularly, the invention relates to hard, dense, low porosity, wear and corrosion resistant coatings containing ultrafine particles of a transition metal boride dispersed in a metallic matrix. The invention also relates to a process for preparing such coatings in situ by thermal spray and diffusion reaction techniques.
Throughout the specification, reference will be made to plasma arc spraying and detonation gun (D-Gun) techniques for producing coatings. Typical detonation gun techniques are disclosed in U.S. Pat. Nos. 2,714,563 and 2,950,867. Plasma arc spray techniques are disclosed in U.S. Pat. Nos. 2,858,411 and 3,016,447. Other thermal spray techniques are also known, for example, so-called "high velocity" plasma and "hypersonic" combustion spray processes, as well as the various flame spray processes. Heat treatment of the coatings is necessary and may be done after deposition in a vacuum or inert gas furnace or by electron beam, laser beam, induction heating, transferred plasma arc or other technique. Alternative deposition techniques such as slurries, filled fabrics or electrophoresis, followed by heat treatment, are also known. Still other methods include simultaneous deposition and fusion utilizing plasma transferred arc, laser or electron beam surface fusion with or without post deposition heat treatment.
2. Background Art
Coatings containing transition metal borides are known in the art. The most common coatings are those produced by thermal spraying so-called "self-fluxing" Ni--Cr--B--Si--Fe alloys. These coatings contain low volume fractions of the boride (i.e. less than 25 vol. %). The metal borides used in the coating have been predominantly chromium borides.
Coatings have also been prepared by flame spraying powder mixtures of a transition metal carbide and a brazing alloy e.g. AMS 4777 (AWS BNi-2), onto a substrate. The so-prepared coatings contain essentially unreacted metal carbide in an alloy matrix. The matrix is usually precipitation strengthened with a low volume fraction of a transition metal boride, e.g., CrB. The total coating composition is essentially the same whether the coating is employed as-deposited or after post-coating fusion, except for minor interdiffusion with the substrate during heat treatment.
U.S. Pat. No. 4,173,685 issued to M. H. Weatherly on Nov. 6, 1979, discloses high-density wear and corrosion resistant coatings prepared by first depositing onto a substrate a coating having an as-deposited density greater than 75% of theoretical by methods such as plasma spray. The powder composition comprises two or more components, the first component containing a metal carbide such as tungsten, chromium or molybdenum carbide, and optionally a binder, e.g., nickel, iron or cobalt, and the second component containing an alloy or alloy mixture containing boron, e.g., Ni--B--Cr--Fe--Si. The first component constitutes 40 to 75 weight percent of the entire composition. The as-deposited coating is then heated to a temperature greater than about 950.degree. C. for a period of time sufficient to cause substantial melting of the second component and reaction of the second component with a substantial portion of the first component. The coating is then cooled allowing the formation of borides, carbides, and intermetallic phases resulting in a hard, dense coating.
The microstructures of coatings prepared according to the Weatherly patent consist of fairly coarse, hard, acicular particles of metal carbide dispersed in a metal matrix. Although these coatings exhibit excellent wear properties, there are applications where the coatings cannot be used successfully because the carbide particles are too abrasive and result in excessive wear of mating components. Moreover, the coating and substrate when heat treated often expand or contract at different rates and this can result in undesirable microcracks or even spalling. Furthermore, due to interdiffusion reactions occurring between the coating and certain stainless steel substrates, chromium-rich carbides precipitate at grain boundaries and within the grains of the steel resulting in sensitization and loss of corrosion resistance.